A spring brake chamber is adopted as a drive device in a brake device of a large vehicle such as a bus, truck, trailer, and the like (refer to, for example, patent documents 1 and 2).
The spring brake chamber includes a cylindrical primary chamber and a cylindrical secondary chamber, which is coupled to the primary chamber. The two ends of the primary chamber are closed. One end of the secondary chamber is open, and the other end is closed. One end of the secondary chamber, that is, the end of the secondary chamber opposite to an end closer to the primary chamber, includes a cylindrical rod insertion portion, into which a second push rod is inserted.
The spring brake chamber includes a cylindrical first case forming the primary chamber, a cylindrical second case forming the secondary chamber, and a coupling case that forms the primary chamber in cooperation with the first case and forms the secondary chamber in cooperation with the second case. The first case includes an open end and a closed end. The two ends of the second case are both open. The coupling case, which has an I-shaped cross-section, forms the primary chamber by closing the first case and forms the secondary chamber by closing the second case. The first case and the coupling case are fixed together when clamped by a clamp ring or when the open end of the first case is crimped. Furthermore, the second case and the coupling case are clamped and fixed by a clamp ring or when the open end of the second case is crimped.
A cylindrical first piston is accommodated in the primary chamber so as to be movable in the axial direction. The first piston includes an open end and a closed end. A compression spring that biases the first piston toward the secondary chamber is arranged in the primary chamber. The first piston partitions the primary chamber into two compartments, namely, a spring compartment that accommodates the compression spring and a first control compartment that is closer to the coupling case. A first push rod, which extends through the coupling case, is fixed to the first piston. Thus, the first push rod is movable back and forth in the axial direction integrally with the first piston. A diaphragm is arranged in the secondary chamber to partition the secondary chamber into two compartments, namely, a second control compartment that is closer to the coupling case and a piston compartment that accommodates a second piston rod. The entire circumference of the diaphragm is fixed to an inner wall of the secondary chamber. A second piston is arranged in a space closer to the rod insertion portion in the secondary chamber. The second piston moves in the rod insertion portion integrally with the diaphragm when the diaphragm moves. The second piston includes a disc, which is attached to the diaphragm, and a bar-shaped second push rod, which moves in the rod insertion portion.
A wedge that extends a brake shoe of a brake drum is connected to the distal end of the second piston. In the spring brake chamber, compressed air is supplied to at least one of the first control compartment of the primary chamber and the second control compartment of the secondary chamber to drive the wedge and apply the brake.